It is sometimes necessary to periodically check the status of fasteners used to clamp structural joints. For example, in the case of vehicles such as aircraft, the clamping force applied by fasteners in certain mission critical structural joints and assemblies must be maintained within prescribed limits. Periodic monitoring of fastener clamping force may be necessary because of the tendency of some structures to relax over time, and physical changes in the fasteners such as work hardening and creep, all of which may result in a change in the preload applied by a fastener, and thus the stress on the fastener. Where fastener preload is found to be outside of the prescribed limits, it may be necessary to re-torque the fastener, or replace it. In the past, monitoring such fasteners was performed by maintenance technicians who would physically check and record the status of the fasteners, including the level of fastener preload. This manual process was time-consuming and labor intensive, and could sometimes be difficult to perform where the fasteners were located in areas not easily accessed.
More recently, fasteners have been devised that incorporate a sensor which measures the stress, and thus the preload, on the fastener. However, in order to read the measured preload and associate it with the particular fastener being read, a technician must physically contact the fastener with a reader that reads the preload as well as a bar code that uniquely identifies the fastener. This approach to collecting fastener data can also be time-consuming, labor intensive, and difficult to perform where fastener access is limited. In some cases, where fasteners are located within a sealed-off area or a particularly small compartment, such as the tail section of an aircraft, it may not be possible to make physical contact with the fastener, thus precluding measurement of fastener preloads.
Accordingly, there is a need for a non-contact method and apparatus for remote collection of data from fasteners, such as the preload or stress on a fastener. There is also a need for a method and apparatus that may be adapted for monitoring existing types of fasteners, even where it may be difficult or impossible to physically access the fasteners.